Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 39

CRUSTAL STRUCTURE OF ICELAND
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4 fcm/sec to the east, south and west. It is perhaps significant that
the low-velocity area appears to coincide with the epicenter area of
m°st of the largest known earthquakes in southern Iceland (Stefáns-
son, 1967). No similar trends are obvious in other parts of Iceland.
6.3. SW-Iceland.
As used here this area comprises the Reykjanes peninsula and
extends to the Hengill area in the northeast.
Four refraction profiles exist in this area, two of them reversed.
In addition three long profiles run from a common shot point in
lake Graenavatn. These are the Graenavatn—Eyjafjöll profile (L3),
which was partly discussea under the S-Iceland section, and the pro-
files CP and WP, that were measured hy Báth in 1959. Further-
more, an attempt was made to find structural variations across the
volcanic zone by an arc shooting procedure from the same shot point
m Graenavatn. Seismometer stations for successive shots were located
along the main highway from Reykjavík to Thorlákshöfn at a dis-
tance from the shot point that varied between 28 and 36 km. The
total delay times for the P3-wave on profiles in SW-Iceland are given
m Fig. 10, and for the arc profile in Fig. 11.
At the southwestern tip of the Reykjanes peninsula two profiles
run from a common shot point towards north and east. The Reykja-
nes-Gardskagi profile (46a and 46b) runs north to a distance of
about 29 km and is reversed. The Reykjanes—Krísuvík profile (44)
ls about 19 km long and unreversed. This profile had to be discon-
tmued at this distance due to unsatisfactory radio connection with
the shot point station.
The structure near the Reykjanes shot point is computed on the
basis of travel times of both profiles Reykjanes-Gardskagi and Reykja-
nes-Krísuvík. The surface low-velocity layer appears to be composed
°f two layers, an upper one 0.2 km thick with a P-wave velocity of
2.2 km/sec, and a lower one 0.7 km thick with a velocity of 3.0
km/sec. The underlying layer 1 has a velocity of 4.2 km/sec. The
delay time for the P3-wave is 0.58 sec, which gives a thickness of
layer 1 of 1.7 km. Total depth to layer 3 is then 2.6 km.
The profile Reykjanes—Krísuvík crosses the high-temperature
thermal area at Reykjanes. A slight time delay of some 0.05 sec rela-
tive to neighbouring stations was observed at a distance of 3.35 km
east of the shot point. This delay could be due to lower velocities